Phenotype-first hypotheses, spandrels and early metazoan evolution

Area of Science:

• Evolutionary biology
• Developmental biology
• Genetics

Background:

• The neo-Darwinian paradigm posits that genetic mutations are the primary drivers of evolutionary variation.
• A phenotype-first approach suggests that developmental processes themselves can generate novel variations.
• Richard Watson's 'developmental memory' concept offers one such mechanism, particularly relevant for later metazoan evolution.

Purpose of the Study:

• To explore alternative sources of phenotypic variability beyond genetic factors.
• To investigate the role of developmental processes in early metazoan evolution.
• To propose an extension of the phenotype-first hypothesis applicable to early evolutionary stages.

Main Methods:

• Interpretation of Stuart Newman's account of deep metazoan phylogenesis.
• Analysis of developmental processes as reservoirs of phenotypic variability.
• Theoretical synthesis of existing hypotheses on evolutionary novelty.

Main Results:

• Developmental processes, specifically 'spandrels,' represent a significant source of phenotypic variability.
• Spandrels offer a plausible mechanism for generating variation in early metazoans, where 'developmental memory' is less applicable.
• The concept of spandrels complements and extends the phenotype-first hypothesis.

Conclusions:

• Phenotypic variation can arise independently of genetic mutation, challenging central tenets of neo-Darwinism.
• 'Spandrels' are identified as a crucial, previously underappreciated source of evolutionary novelty.
• Gerd Müller's 'side-effect hypothesis' provides a unifying framework for this expanded phenotype-first perspective.